A hypothesis for the quiescence during the pregnancy will be investigated. The hypothesis is that myometrial cyclic guanosine 3',5' (cGMP) is the cellular mediator of the uterine quiescence and that its productions is stimulated by a C-Natriuretic Peptide (CNP)-like substance produced by the fetal chorionic membrane. Further is hypothesized that the effect of this factor is offset by an endogenous inhibitor (EI-cGMP) circulating in the maternal and fetal plasma. That requires that the onset of normal labor follows an increase in EI-cGMP and/or a decrease in CNP resulting in a decline in myometrial cGMP. A lack of coordination between these factors could lead to idiopathic preterm birth. Portions of the working hypothesis will be tested by investigating the regulation of myometrial cGMP synthesis and degradation, specifically: (1) measure human myometrial cGMP content across pregnancy by RIA using myometrial samples collected from patients at different gestational ages and labor status; (2) determine the principal mechanism of myometrial cGMP synthesis using a guinea pig model. There are two candidate pathways: natriuretic peptide stimulation of particulate guanylate cyclases and nitric oxide (NO) stimulation of soluble guanylate cyclase. Based on preliminary study, the focus will be centered on CNP whose pathway will be blocked using a monoclonal antibody targeted against its receptor (particulate guanylate cyclase type B). The NO dependent pathway will be inhibited using the nitric oxide synthase or soluble guanylate cyclase inhibitors. In both and in a control group, the evolution of the pregnancy and cGMP myometrial content will be studied. (3) CNP production will be measured by RIA in human chorion samples collected from patients at different gestational ages and labor status. The regulation of its production will be tested using human chorion cultured cells evaluating the CNP and CNP mRNA levels in control conditions and under the stimulus of known regulatory agents such as steroids and cytokines; (4) The EI-cGMP activity will be determined in human plasma samples from women of differing gestational ages and labor status looking for changes induced for the plasma in basal or CNP stimulated cGMP content in human myometrial cultured cells. The chemical nature, identity and action mechanism of EI-cGMP will be elucidated. (5) The specific phosphodiesterase (PDE) family will be identified in human myometrium by Western Blot, and changes in its expression during the course of the gestation measured in human myometrial samples. In addition, the time course of the pregnancy and the cGMP myometrial levels will be evaluated in pregnant guinea pig following the inhibition of PDE using the most specific inhibitors available. The proposed studies will provide important new information that may aid the development of new strategies for the management of preterm labor, a main cause of perinatal morbidity and mortality.